1. Field of the Invention
The invention is related generally to oil field exploration and in particular to use of multi-component induction measurements for identifying fractures in surrounding formations.
2. Description of the Related Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, are diagnostic of the petrophysical and other properties of the formation and the fluids therein.
In thinly laminated sand-shale sequences or shaly-sand formations, the formation electrical anisotropy becomes important in determining the hydrocarbon saturation from the resistivity measurements. Due to physical complexities of the formation, determining the electrical anisotropy can be complicated.
For example, intrinsic and macroscopic properties may give rise to anisotropy (i.e., directional dependence). Fractures may also have a substantial effect. In some instances, fractures occur as a result of drilling processes. Accordingly, fracturing as a result of drilling can affect other measurements. Unfortunately, techniques presently available do not adequately provide for characterization of anisotropy while drilling.
When drilling a well it is critical to keep the pressure in the well bore resulting from the weight of the drilling mud below the formation fracture pressure. If the pressure in the well does exceed the formation fracture pressure then the well will fracture and the drilling mud will flow out of the well. Once the drilling mud flows out of the well shallower formations may flow high pressure water or hydrocarbons back to the surface and the well will blow out.
Generally, principles of geomechanics are used to determine the appropriate mud weight to prevent high pressure formation fluids from flowing to the surface (minimum mud weight) as well as a maximum mud weight to keep from fracturing the formation. Unfortunately the actual mud weight includes both static and dynamic components. The static mud weigh is controlled by weighting agents in the mud and suspended cuttings. The dynamic component is due to the movement of the drilling mud and drill pipe (both rotation and upward and downward motion) which can increase the apparent mud weight above the static mud weight.
It is the equivalent circulating density of the mud which is critical. There are well known rules for calculating the ECD, but these may not be accurate and the strength of the formation is not know precisely. Hence during drilling the formation may fracture (drilling induced fractures). These drilling induced fracture should be avoided. Knowing in real time that the formation is beginning to fracture would allow one to reduce the mud weight and thus avoid “fraccing” (i.e., factoring) the well and a subsequent potential blow out.
There is a need for advanced methods for providing determinations of formation anisotropy. Preferably, the methods should consistently provide accurate and reliable data, be computationally efficient and useful for implementation while drilling a wellbore. Ultimately, the methods provide users with a capability to one to reduce the mud weight and avoid fraccing the well and experiencing a blow out.